1. Field of the Invention
This invention relates to a method of controlling an induction motor and apparatus therefor, and particularly to a method of controlling an induction motor and apparatus therefor in which torque control can be attained with high accuracy.
2. Description of Prior Art
There have been proposed two methods of controlling an induction motor wherein the control is made at the primary and at the secondary side, respectively. For example, U.S. Pat. No. 3,824,437 (corresponding to West German Laid-open Specification No. 1,941,312) discloses an induction motor controlling method which belongs in the category of the primary-side controlling method and which is called the vector control in which the primary current of the induction motor is decomposed into an exciting component which is the component in the same direction as that of the axis of magnetic flux and an effective component which is the component perpendicular to the exciting component, so that the respective components can be controlled separately. By this vector control, the torque control in the induction motor can be attained with high accuracy as in the DC motor.
In this vector control, it is necessary to detect the flux axis position as a reference signal for the decomposition of the primary current into its components. The flux axis position can be detected by a magnetic flux detector mounted on the induction motor.
However, the induction motor must be provided with such a flux detector, and therefore this vector control cannot be adopted in the general-purpose motors. In addition, there is a practical problem of providing a signal line for transmitting the output signal of the flux detector to the control apparatus.
In order to solve this problem, there has been proposed a method of calculating and detecting the flux axis position within the control apparatus, as, for example, disclosed in the specification of Japanese Patent Application Laid-open No. 11125/1976.
As will be described in detail later, this method, however, has a problem that the speed control is unstable at high operating frequencies so that a predetermined speed response can not be obtained and therefore accurate speed control can not be attained.